Method of manufacture of slow-wave structures



Dec. 22, 1959 A. H. IVERSEN 2,918,358

METHOD OF MANUFACTURE OF SLOW-WAVE STRUCTURES Filed Dec. 51, 1956 F /'g. l.

INVENTOR. Arthur H. Iversen,

AGENZ misses; Patented Dec. 22, 1959 United States Patent Ofifice METHOD OF MANUFACTURE OF SLOW-WAVE STRUCTURES Arthur H. Iversen, Santa Monica, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application December 31, 1956, Serial No. 631,842

7 Claims. c1. 41 43 This invention relates generally to traveling-wave tubes and more specifically to a slow-wave structure for use therein, and a method of manufacturing such slow-wave structures.

Traveling-wave tubes generally comprise an evacuated envelope, a slow-wave structure disposed within the envelope for propagating electromagnetic waves at a velocity substantially less than the velocity of light, and an electron gun disposed at one end of the envelope for projecting an electron stream in energy exchange relationship with the traveling waves propagated along the slow-wave structure.

Heretofore, the slow-wave structure adapted for propagating the electromagnetic waves in a traveling-wave tube has been manufactured by precision milling of slots in a cylindrical metallic tube, by actual winding of a wire helix on a mandrel, by complex metallic stampings, and by other precision machining techniques which require great skill, elaborate equipment and considerable time. The prior art techniques have also required that after construction of the slow-wave structure, it be thereafter enclosed in an evacuated envelope. This gave rise to many problems of supporting the slow-wave structure within the envelope while maintaining critical distances between the slow-wave structure and the envelope and aligning the slow-wave structure with the electron stream. In some priormethods of supporting slow-wave structures, a serious problem of the envelope causing dielectric loading of the slow-wave structure was also encountered, thereby decreasing the efficiency of the tube. Further, in accordance with past techniques, it was very difficult to change the type of metal along the length of the slowwave structure so as to vary the resistance of the slowwave structure at desired points along the length of the slow-wave structure.

It is therefore an object of the present invention to provide a method of manufacturing slow-wave structures which is not subject to the disadvantages of the prior art.

It is another object of the present invention to provide a method of manufacturing slow-wave structures whereby the envelopeof the traveling-wave tube serves to support the slow-wave structure but with a minimum of dielectric loading of the slow-wave structure.

It is a further object of the present invention to provide a method of manufacturing slow-wave structures of desired configurations and providing variations in metal types and electric conductivity along the length of the slow-wave structures.

It is still another object of the present invention to provide slow-wave structures of desired configurations wherein the envelope of the traveling-wave tube supports the slow-wave structure, portions of the inner surface of the envelope intermediate the slow-wave structure being removed to reduce dielectric loading.

In accordance with the present invention, these objects are achieved by applying a suitable metallic powder suspension to the innersurface of a cylindrical glass tube in a predeterminedslow-wave structure pattern. The metalagent which does not attack the metal.

lie slow-wave structure deposit is then fused to the inner surface of the tube. The outer surface of the tube may be coated with an appropriate protective agent, and portions of the glass of the tube around the slow-wave structure deposit are then etched away by a suitable etching in this manner dielectric loading of the resulting traveling-wave tube is reduced.

The novel features which are believed to be characteristic of the invention both as to its organization and method of operation, together with further objects and advantages thereof will be better understood from the foliowing description considered in connection with the accompanying drawing in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawing is for the purpose of illustration and description only and is not intended as a definition of the limits of the invention.

Fig. l. is a side elevational view illustrating a method of applying a metallic suspension to the inner surface of a glass envelope and associated equipment in accordance with the present invention for use in a traveling-Wave tube;

Figs. 2 and 3 arecross-sectional views showing the glass envelope and slow-wave structure before and after the glass intermediate the slow-wave structure has been removed;

Fig. 4 is a sectional view in perspective of another configuration of a slow-wave structure and associated equip ment constructed in accordance with an alternative embodiment of the present invention; and

Fig. 5 is a side elevational view of a traveling-wave tube embodying a helix or slow-wave structure of the invention.

Referring now to Fig. 1 of the drawing, a machine such as a lathe 10 is shown supported by a frame 11. Mounted on frame 11 is a motor 14 driving a chuck 13, motor 14 being capable of moving a carriage 15 transversely along supporting frame 11 by means of a lead screw 12 at a rate in accordance with adjustable controls 21. Mounted on top of carriage 15 is a feed jar 16 which moves with carriage 15 while permitting the contents of feed jar 16 to flow past a valve 17 through a feed tube 18 and eventually discharge from orifice or nozzle 19.

To apply a slow-wave structure pattern, as for example a helix, to the inner surface of a glass cylinder or tube 20, tube 20 is inserted in chuck 13, and controls 21 are set toobtain the desired rate of feed of carriage 16 and the rate of rotation of chuck 13 and hence glass tube 20. A desired metallic powder suspension, such as a 425 mesh of platinum, tungsten, or molydenum, or a mixture of metals such as molydenum and manganese in a suspension agent such as acetone and nitro cellulose, is placed in feed jar 16 which may then be pressurized to improve the flow of metallic suspension if gravity flow should be insufficient.

Motor 14 is started and valve 17 of feed tube 18 is opened. As carriage 15 moves transversely along lead screw 12 and as glass tube 20 rotates, the metallic powder suspension is deposited on the inner surface of glass cylinder 20 through orifice 19 thereby forming a desired helix pattern. If a single helix is desired, valve 17 of feed tube 18 is closed when a single traverse is completed and the carriage reversed so as to withdraw feed tube 18. If a contrawound helix is desired, valve 17 is allowed to remain open after the initial deposit of the metallic powder suspension along the length of glass cylinder 20 is made during a traverse of the carriage, and the carriage reversed thereby forming a contrawound helix.

.After the desired slow-wave pattern has been applied to the inner surface of glass cylinder or tube 20, the

. tube is removed from chuck 13 and the suspension agent of the metallic suspension removed, as by evaporation or heating. The metallic slow-wave structure pattern is then fused, as by heating, to the inner surface of cylinder 20.

Fig. 2 represents a metallic helix or slow-wave structure 22 fused to the inner surface of cylinder 20 wherein cylinder 20 serves to support the single helix formed by the metallic deposit.

As shown in Fig. 3, glass portions 23 adjacent successive helix loops 22 have been removed, such as by etching, with hydrofluoric acid, in order to reduce dielectric loading caused by close association of glass cylinder 20 with helix slow-wave structure 22. Where the entire glass cylinder 20 is immersed in the etching agent during the etching process, the outer surface of cylinder 20 may be protected against etching by a protective agent 24 such as a cellulose plastic. Otherwise, it may be sufficient to stopper one end of glass cylinder 20 with a stopper of paraffin or cellulose plastic and pour the etching agent directly into glass cylinder 20.

Fig. 4 illustrates another embodiment of the present invention wherein it is possible to fabricate a slow-wave structure of two metals of different conductivity for use in traveling-wave tubes wherein it is desired to attenuate undesirable frequencies in certain areas or regions along the length of the slow-wave structure. Furthermore, a metal of low conductivity may be used to provide attenuations for electromagnetic waves which may be reflected at the impedance match between the helix and the output waveguide. The method is similar to the above disclosed method of fabricating a slow-wave structure of a single metal.

Feed jar 28 contains a metallic powder suspension of one conductivity, such as molydenum, and is equipped with a feed tube 32 and a valve 30. Adjacent feed jar 28 is a feed jar 27 containing a metallic powder suspension of a different conductivity, such as platinum, which is also equipped with a feed tube 33 and a valve 29. Both feed tubes 32 and 33 join feed tube 31 which in turn conducts the flow of one or both metallic powder suspensions and discharges the suspension from orifice 34 for deposit along the length of glass cylinder 20.

In order to form the slow-wave structure illustrated in Fig. 4, the lathe, described in reference to Fig. 1 may be used. After inserting a glass cylinder 20 in the lathe chuck as before, orifice 34 of feed tube 31 is inserted in glass cylinder 20. Valve 30 is opened permitting the contents of feed jar 28, such as a platinum suspension, to flow through feed tube 32 into feed tube 31. Glass cylinder 20 is rotated without advancing the feed tube 31 thereby forming a circular band or ring 35 of the metallic powder suspension contained in feed jar 28. Since it is desirable to have the transition from one metal to another be gradual in order to minimize deleterious reflections of electromagnetic waves due to abrupt changes in metals, valve 29 is opened without closing valve 30 permitting the contents of feed jar 27, such as a molydenum suspension, to flow through feed tube 33 into feed tube 31 and thereby mix with the contents of feed jar 28. Feed tube 31 is then advanced a predetermined length while glass cylinder 20 remains stationary, thereby forming a longitudinal strip 36 which will consist of a mixture of both metallic powder suspensions. At this time, valve 30 is closed and glass cylinder 20 rotated without moving feed tube 31 to form a circular band 26 composed of the metallic powder suspension contained in feed jar 27, which may be a molydenum suspension. To form longitudinal element 25, feed tube 31 is again advanced for a predetermined length into glass cylinder 20 to deposit a longitudinal element 25 also composed, for example, of a molydenum powder suspension contained in feed jar 27. To deposit again the metallic powder suspension of the metal contained in feed jar 28, and thereby obtain its high conductivity, valve 29 is closed and the transition from the metallic powder suspension contained in feed jar 27 to the metallic powder.- sus- '4 pension contained in feed jar 28. This procedure may be repeated along the length of the slow-wave structure to obtain any desired pattern. The glass adjacent the fused slow-wave structure is then removed, as by etching, to reduce dielectric loading associated with the slow-wave structure and tube.

Fig. 5 illustrates a traveling-wave tube 37 embodying a contrawound helix or slow-wave structure'in accord ance with the present invention. Enclosed in the enlarged left hand portion of envelope 38 is an electron gun comprised of a filament 39 connected to a battery or voltage source 40 by leads 41 and 42. Filament 39 heats a cathode 44 thus providing a source of electrons which are focused in a beam by a focusing electrode 45 which may have the shape of a truncated cone. The electrons thus emitted and formed into a beam are accelerated by accelerating electrodes 46 and 47 in a conventional manner. Accelerating electrodes 46 and 47 are connected by leads 48 and 49 respectively to appropriate sources of accelerating potential such as different taps on a battery 43.

For purposes of coupling electromagnetic energy to the slow-wave structure, the inner conductor of a coaxial input transmission line 51 is connected to the electron gun end of a contrawound helix 50 used for propagating electromagnetic waves at an axial velocity less than the speed of light. Envelope 38 which. supports the contrawound helix 50 and aligns it with the electron beam also serves as the evacuated envelope of traveling-wave tube 37. The output or collector end of contrawoundhelix 50 is connected to the inner conductor of a coaxial output transmission line 52. The electron beam is collected by collector electrode 53 which is connected to a positive source of potential such as a battery 54 for purposes of minimizing the deleterious effects of secondarily emitted electrons. As shown in Fig. 5, portions of the inner glass surface 55 of envelope 38 adjacent contrawound helix 50 have been removed, as by etching, to reduce dielectric loading associated with the contrawound helix and envelope. A matching ferrule such as shown at 56 may be provided at the input and output ends of the contrawound helix 50. If desired, the ferrules may be applied by a metallic powder suspen sion in the same manner as is the helix.

It is seen that there has been disclosed a method of manufacturing a variety of slow-wave structures without the necessity of precision milling, complex metallic stampings, and precision machining techniques requiring great skill and elaborate equipment. The method also provides a slow-wave structure wherein the envelope serves to support the structure while acting as the evacuated envelope of a traveling-wave tube. The disclosed method in addition, eliminates the problem of enclosing the slow-wave structure in a separate evacuated envelope after manufacture of the slow-wave structure as a special step. The problem of aligning the slow-wave structure and envelope with the electron stream while maintaining critical distances between the slow-wave structure and envelope is also eliminated.

The slow-wave structure manufactured by this method has the advantage not only of simplicity, but has the important feature of minimum dielectric loading thereby increasing traveling-wave tube efficiency by raising the power output. The disclosed method also permits the manufacture of slow-wave structures of metals of different conductivity deposited along predetermined regions of the length of the slow-wave structure.

What is claimed is:

1. A method of manufacturing a slow-wave structure adapted for use in a traveling-wave tube comprising the steps of: applying a metallic powder to a limited predetermined area of the inner surface of a tube in the configuration of a predetermined slow-wave structure; fusing the applied metallic powder to said inner surface of said tube; and removing a portion of. said inner surface of said tube adjacent said fused metallic powder, thereby to reduce dielectric loading associated with said slow-wave structure and tube.

2. A method of manufacturing a slow-wave structure adapted for use in a traveling-wave tube comprising the steps of: applying a metallic powder suspension to a limited predetermined area of the inner surface of a tube in the configuration of a predetermined slow-wave structure, fusing the applied metallic powder suspension to said inner surface of said tube, and removing a portion of said inner surface of said tube adjacent said fused metallic powder, thereby to reduce dielectric loading associated with said slow-wave structure and tube.

3. A method of manufacturing a slow-wave structure adapted for use in a traveling-wave tube comprising the steps of: applying a metallic powder in a suspension agent to a limited predetermined area of the inner surface of a tube to provide a predetermined slow-wave structure, removing the suspension agent, fusing the applied metallic powder to said inner surface of said tube, and removing a portion of said tube along said inner surface and adjacent said fused metallic powder suspension, thereby to reduce dielectric loading associated with said slow-wave structure and tube.

4. A method of manufacturing a slow-wave structure adapted for use in a traveling-wave tube comprising the steps of: rotating a cylindrical glass tube about its axis; applying a platinum powder in a suspension agent to a continuous predetermined area of the inner surface of said tube to provide a predetermined slow-wave structure pattern, removing the suspension agent, fusing the applied platinum powder to the inner surface of said tube, etching the inner surface of said tube with hydrofluoric acid to remove portions of said tube about said metallic slow-wave structure, thereby to reduce dielectric loading associated with said slow-wave structure and cylindrical glass tube.

5. A method of manufacturing a slow-wave structure adapted for use in a traveling-wave tube comprising the steps of: rotating a cylindrical glass tube about its axis, applying a platinum powder suspension to a limited area of the inner surface of said tube in a contrawound slowwave structure pattern, heating said platinum powder suspension to simultaneously eliminate the suspension agent and fuse said platinum powder to the inner surface of said tube, etching the inner surface of said tube with hydrofluoric acid to remove portions of said tube about said platinum contrawound slow-wave structure while protecting the outer surface of said tube from the action of the acid with a cellulose plastic coating, thereby to reduce dielectric loading associated with. said slow-wave structure and tube.

6. A method of manufacturing a slow-wave structure adapted for use in a traveling-wave tube: comprising the steps of: rotating a cylindrical glass tube about its axis, applying two metallic powders of different conductivity in predetermined sequence along the inner surface length of said tube in the configuration of a predetermined slowwave structure, fusing the two applied metallic powders to the inner surface of said tube, thereby forming a slowwave structure of a varying conductivity along its length, and removing portions of said tube in areas intermediate the metallic slow-wave structure, thereby to reduce dielectric loading associated with said slow-wave structure and glass tube.

7. A method of manufacturing a slow-wave structure adapted for use in a traveling-wave tube comprising the steps of: rotating a cylindrical glass tube about its axis while applying a metallic powder in a suspension agent of one conductivity followed in predetermined sequence by another metallic powder in a suspension agent of a different conductivity to provide a gradual transition from one metallic powder to the other in the configuration of a predetermined slow-Wave structure, removing the suspension agent, fusing the applied metallic powders to said inner surface of said tube, etching said inner surface of said tube in areas intermediate said metallic slow-wave structure, to remove portions of said tube in said areas, thereby to reduce dielectric loading associated with said slow-wave structure and tube.

References Cited in the file of this patent UNITED STATES PATENTS 1,256,599 Schoop Feb. 19, 1918 2,557,983 Linder June 26, 1951 2,559,389 Beeber et al July 3, 1951 2,587,568 Eisler Feb. 26, 1952 2,602,731 Nierenberg July 8, 1952 2,758,241 Robinson Aug. 7, 1956 2,771,565 Bryant Nov. 20, 1956 2,790,926 Morton Apr. 30, 1957 FOREIGN PATENTS 685,752 Great Britain Jan. 14, 1953 

1. A METHOD OF MANUFACTURING A SLOW-WAVE STRUCTURE ADAPTED FOR USE IN A TRAVELING-WAVE TUBE COMPRISING THE STEP OF: APPLYING A METALLIC POWDER TO A LIMITED PREDETERMINED AREA OF THE INNER SURFACE OF A TUBE IN THE CONFIGURATION OF A PREDETERMINED SLOW-WAVE STRUCTURE FUSING THE APPLIED METALLIC POWDER TO SAID INNER SURFACE OF SAID TUBE; AND REMOVING A PORTION OF SAID INNER SURFACE TACE OF SAID TUBE ADJACENT SAID FUSED METALLIC POWDER, SLOW-WAVE STRUCTURE AND TUBE. 